Equine Protozoal Myeloencephalitis (EPM) is a common cause of neurologic disease in New World horses. It is caused by a parasite termed Sarcocystis neurona (S. neurona), an obligatory intracellular apicomplexan parasite whose multi-phase life cycle is completed in either one or two hosts. S. neurona is known to cycle naturally between opossums and both none-banded armadillos and striped skunks. Horses typically become infected by consuming infectious parasite stages found in opossum feces. Once a horse has been infected, S. neuronacan travel to the brain and spinal cord, where merozoite stages of this parasite replicate and cause pathology.
Horses with EPM typically present with lameness, but may alternatively or additionally present with symptoms characteristic of primary brain disease. Because the parasite can inhabit any area of the central nervous system (CNS) of the horse, symptoms associated with EPM can vary widely. The degree of infection can range from subtle to severe and can involve the brain and/or the spinal cord. EPM is usually progressive.
Presently, a definitive diagnosis of EPM is made by post-mortem examination, where S. neurona organisms are identified in histological lesions. The organ may also be cultured from the lesion. The presence of the organism in the histologic section or when cultured from the lesion establishes the diagnosis. Heretofore, pre-mortem methods for diagnosing EPM were based on assays using whole merozoites, and not a purified protein, to probe for the presence of anti-S. neurona antibodies (as an indication of infection) in the horses. The use of such whole merozoites results in significant cross-reaction with non-S. neurona specific antibodies (e.g., those against other Sarcocystis species). This cross-reactivity obscures interpretation of results using whole merozoite-based assays.